High-density microwave or millimeter-wave circuitry is often photolithographically fabricated upon a semiconductor substrate. Gallium arsenide (GaAs) is ordinarily the semiconductor of choice, offering significant increases in gain over other semiconductors (e.g. silicon) at the desired frequencies.
Several problems arise in the use of gallium arsenide substrates. As a material, gallium arsenide has a high frangibility. This high frangibility leads to an increase in wafer breakage during the circuit fabrication process, hence reducing the effective circuits-per-wafer yield. This is especially pronounced for large circuits having low initial circuit-per-wafer densities.
High frangibility also means that large gallium arsenide circuits are more likely to suffer damage from shock and vibration than are similar circuits in other materials. This can become a limiting factor in the design of devices which must be able to tolerate high G-forces (such as handheld telephones, which may be dropped) and extremes of pressure and vibration (such as a satellite during launch).
Gallium arsenide also suffers from poor thermal conductivity. Poor thermal conductivity requires that gallium arsenide substrates be thin to allow for adequate heat sinking and power dissipation. Making a given gallium arsenide substrate thin, however, exacerbates the specific frangibility of that circuit, and increases the possibility of device failure.
Among semiconductors, gallium arsenide is inherently expensive. Also, the fabrication techniques required of gallium arsenide are themselves more expensive than those of other semiconductors. A given gallium arsenide circuit may be sufficiently expensive, compared to a similar circuit in silicon, so as to prohibit fabrication in production quantities. Thus, those applications where the use of gallium arsenide would be most desirable may also be the very applications where the cost of gallium arsenide would severely limit its use. For example, a phased antenna array, having a thousand active elements coupled to a thousand gallium arsenide circuits, may be prohibitively expensive for commercial applications.
What is needed is a way to create circuits with the high gain of gallium arsenide at microwave and millimeter-wave frequencies, while minimizing the effects of the high frangibility and low thermal conductivity of gallium arsenide as well as the material and fabrication costs thereof.